1. Field of the Invention
The present invention relates to a polyamide-imide resin based insulating varnish from which a highly flexible insulation coating can be made, and an insulated wire having good press-formability.
2. Description of the Related Art
Enameled wires are mainly used for coils in electrical equipment such as motors and transformers, and are typically formed by applying an insulating varnish around a conductor wire and baking the varnish. Such enameled wires may undergo severe mechanical stresses during coil winding processes.
An approach for relieving such mechanical stresses is to provide the surface of an enameled wire with more lubricity. And, a method for providing an enameled wire with a lubricative surface is to apply a highly lubricating material on the outermost surface of the wire and bake the applied material. Known examples of such highly lubricating materials include: a lubricant including paraffin and a fatty acid ester as major constituents, and an insulating varnish containing such a lubricant; an insulating varnish containing a mixture of a stabilized isocyanate and a lubricant; and an insulating varnish containing a titanate ester (see, e.g., JP-A Hei 9(1997)-45143 and JP-A Hei 7(1995)-134912).
With the recent strong trend towards environmentalism, a demand exists for smaller and more efficient motors and transformers. For example, there are increasing cases in which a high-power motor is mounted in a very small space, particularly in hybrid vehicles. Coils in such small and high-power motors occupy a larger volume in the motor. To fabricate such coils, an enameled wire needs to be wound more compactly (higher compression ratio) than before. In such severe winding processes, enameled wires are often press-formed or compressed to too large an extent to be withstood.
To withstand such severe press-forming processes, insulation coatings with an improved wear resistance over conventional coatings are often used. In many cases, such highly wear resistant insulation coatings are formed from a polyamide-imide resin based insulating varnish. For example, 3,3′-dimethylbiphenyl-4,4′-diisocyanate (TODI) is used as an isocyanate constituent of a polyamide-imide resin based insulating varnish to rigidify the backbone of the polyamide-imide resin in the varnish. This improves the wear resistance of the resulting insulation coating and prevents occurrence of damages (such as cracks) in the coating during press-forming processes. Thus, the press-formability of the resulting insulated wire is enhanced. For example, see JP-B 2936895 and JP-A 2007-270074.
However, the polyamide-imide resin contained in the above-described conventional polyamide-imide resin based insulating varnish has a rigid backbone, and therefore insulation coatings made from such an insulating varnish have the disadvantage of poor flexibility (poor elongation properties).
As described above, during recent demanding press-forming processes, insulation coatings often receive very severe mechanical stresses, and are, as a result, deformed by compression to a very large extent. Therefore, such poorly flexible conventional polyamide-imide resin based insulation coatings as described above are particularly problematic when subjected to such large mechanical stresses because they can be deformed (compressed) only up to a certain limit, and may, above the limit, suffer from damages such as cracks.